The invention relates to a method for balancing an energy storage system, wherein the energy storage system comprises a series circuit of storage modules, wherein the series circuit of storage modules has at least two capacitive storage modules, wherein the capacitive storage modules are connected to a balancing device such that a charge of the capacitive storage modules can be influenced in each case by means of a flow of current between the balancing device and the capacitive storage modules, wherein the capacitive storage modules each have a capacitance. The invention also relates to a control device for carrying out the method and to an energy storage arrangement having a control device.
Balancing an energy storage system involves influencing individual storage modules in respect of their voltage. The individual storage modules are energy storage devices such as, for example, capacitors, in particular double-layer capacitors (ultracaps), or batteries. Capacitors or ultracaps are also termed capacitive energy storage devices. Other energy storage devices which can be assigned to this group of capacitive energy storage devices are, for example, lithium capacitors. Also other hybrid energy storage devices, often termed mixed energy storage devices, which exhibit capacitive behavior at least in some cases, may be categorized as capacitive energy storage devices. Similarly, storage modules made up of these components are termed capacitive storage modules. In the case of the energy storage systems considered, the individual storage modules are connected end to end in what is termed a series circuit. The purpose of balancing is to produce, in an operating state, a predefinable voltage at the terminals of the storage modules. For example, in the charged state, all the storage modules should assume the maximum voltage at their terminals. This prevents individual cells of this series circuit from assuming a higher voltage than others and therefore prevents the maximum permissible voltage of the individual storage module from being exceeded. Exceeding the maximum voltage means overloading of the corresponding storage module and results in significantly accelerated aging of the corresponding storage module. Balancing should take place during normal operation. It is also necessary to bring all the storage modules to zero voltage in the event of complete discharge. Balancing is also used to prevent charge reversal of storage modules to negative voltage during discharging.
The different voltages during operation of the individual capacitive storage modules are caused, among other things, by the storage modules having different capacitances. These may be caused, for example, by aging processes of the capacitive storage modules. The capacitance of storage modules usually decreases with age. The fall-off in capacitance is influenced by operating and environmental conditions, such as voltage and temperature, for example.
A usual balancing method consists of connecting a balancing resistor in parallel with the storage module if a particular threshold value of the voltage of the storage module is exceeded, so as to influence the voltage of the storage module. Thus balancing can be carried out in a state such as maximum charge, for example. In the maximum charge state, all the storage modules are then at maximum voltage. The individual storage modules can also be fully discharged to zero voltage via these balancing resistors.
Circuits are also known in which, for balancing purposes, charge is not supplied from the storage modules to a balancing resistor, but an exchange of charges takes place between the individual storage modules.